Convert number to English words in bash

number_to_words.sh

#!/bin/bash

number_to_words() {
    num=$1

    if [ "$num" -eq 0 ]; then
        echo "zero"
        return 0
    fi

    if [ "$num" -lt 0 ]; then
        echo -n "minus "
        num=$(( -num ))
    fi

    ones=("zero" "one" "two" "three" "four" "five" "six" "seven" "eight" "nine" "ten" "eleven" "twelve" "thirteen" "fourteen" "fifteen" "sixteen" "seventeen" "eighteen" "nineteen")
    tens=("" "" "twenty" "thirty" "forty" "fifty" "sixty" "seventy" "eighty" "ninety")
    scales=("" "thousand" "million" "billion" "trillion" "quadrillion" "quintillion")

    to_words_under_1000() {
        n=$1
        result=""
        
        if [ "$n" -ge 100 ]; then
            result+="${ones[$(( n / 100 ))]} hundred"
            n=$(( n % 100 ))
            [ "$n" -ne 0 ] && result+=" and "
        fi

        if [ "$n" -ge 20 ]; then
            result+="${tens[$(( n / 10 ))]}"
            n=$(( n % 10 ))
            [ "$n" -ne 0 ] && result+="-${ones[$n]}"
        elif [ "$n" -gt 0 ]; then
            result+="${ones[$n]}"
        fi

        echo "$result"
    }

    parts=()
    scale_index=0

    while [ "$num" -gt 0 ]; do
        part=$(( num % 1000 ))
        if [ "$part" -ne 0 ]; then
            words=$(to_words_under_1000 "$part")
            [ "$scale_index" -ne 0 ] && words="$words ${scales[$scale_index]}"
            parts=("$words" "${parts[@]}")
        fi
        num=$(( num / 1000 ))
        scale_index=$(( scale_index + 1 ))
    done

    echo "${parts[*]}"
}

number_to_words 42